c++ - Why does std::forward discard constexpr-ness? -

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being not declared constexpr, std::forward discard constexpr-ness function forwards arguments to. why std::forward not declared constexpr can preserve constexpr-ness?

example: (tested g++ snapshot-2011-02-19)

#include <utility>  template <typename t> constexpr int f(t x) { return -13;} template <typename t> constexpr int g(t&& x) { return f(std::forward<t>(x));}  int main() {   constexpr int j = f(3.5f);   // next line not compile:    // error: ‘constexpr int g(t&&) [with t = float]’ not constexpr function   constexpr int j2 = g(3.5f); }

note: technically, easy make std::forward constexpr, e.g., (note in g std::forward has been replaced fix::forward):

#include <utility>  namespace fix {   /// constexpr variant of forward, adapted <utility>:   template<typename tp>   inline constexpr tp&&   forward(typename std::remove_reference<tp>::type& t)    { return static_cast<tp&&>(t); }    template<typename tp>   inline constexpr tp&&   forward(typename std::remove_reference<tp>::type&& t)    {     static_assert(!std::is_lvalue_reference<tp>::value, "template argument"           " substituting tp lvalue reference type");     return static_cast<tp&&>(t);   } } // namespace fix  template <typename t> constexpr int f(t x) { return -13;} template <typename t> constexpr int g(t&& x) { return f(fix::forward<t>(x));}  int main() {   constexpr int j = f(3.5f);   // compiles fine:   constexpr int j2 = g(3.5f); }

my question is: why std::forward not defined fix::forward ?

note2: question related other question constexpr std::tuple std::forward not being constexpr technical reason why std::tuple cannot created calling cstr rvalues, question here (much) more general.

the general answer c++ committee's library working group have not done exhaustive trawl through working draft looking opportunities use new core facilities. these features have been used people have had time , inclination @ possible uses, there not time exhaustive checking.

there papers regarding additional uses of constexpr in works, such in november 2010 mailing.


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